Boire, D. | Search bioRxiv

bioRxiv2019

Afferent connections of the primary somatosensory cortex of the mouse for contextual and multisensory processing

Sensory information is conveyed from peripheral receptors through specific thalamic relays to primary areas of the cerebral cortex. Information is then routed to specialized areas for the treatment of specific aspects of the sensory signals and to multisensory associative areas. Information processing in primary sensory cortices is influenced by contextual information from top-down projections of multiple cortical motor and associative areas as well as areas of other sensory modalities and higher order thalamic nuclei. The primary sensory cortices are thus located at the interface of the ascending and descending pathways. The theory of predictive coding implies that the primary areas are the site of comparison between the sensory information expected as a function of the context and the sensory information that comes from the environment. To better understand the anatomical basis of this model of sensory systems we have charted the cortical and subcortical afferent inputs in the ipsilateral and contralateral hemispheres of the primary somatosensory cortex of adult C57Bl/6 mice. Iontophoretic injections of the b-fragment of cholera toxin were performed inside the mystacial caudal barrel field, more rostral barrel field and somatosensory cortex outside the barrel field to test the hypothesis that differences exist between these three parts and to compare their projections to the subnetworks built from the Mouse Connectome Project data. The laminar distribution of retrogradely labeled cell bodies was used to classify the projections as feedback, feedforward or lateral. Layer indices range between -1 and 1, indicating feedback and feedforward connections respectively. The primary somatosensory cortex and the barrel field have afferent connections with somatosensory areas, non-somatosensory primary sensory areas, multisensory, motor, associative, and neuromodulatory areas. The caudal part of the barrel field displays different and more abundant cortical and subcortical connections compared to the rest of the primary somatosensory cortex. Layer indices of cortical projections to the primary somatosensory cortex and the barrel field were mainly negative and very similar for ipsilateral and contralateral projections. These data demonstrate that the primary somatosensory cortex receives sensory and non-sensory information from cortical and subcortical sources.

neuroscience↗

bioRxiv2019

Effects of enucleation on the direct reciprocal corticocortical connections between primary visual and somatosensory cortices of the mouse

Multisensory convergence is present in the cerebral cortex even at the initial stages of processing in the primary and low-level sensory cortices. Direct connections between primary sensory cortices are a particular feature of rodent cortical connectivity. Our previous studies have shown asymmetric projections between the primary visual and somatosensory cortices in mice. Binocular enucleation produces a relative reduction of the projections from the somatosensory cortex to the visual cortex in mice. The purpose of this study is to compare the direct reciprocal cross-modal connections between the primary visual (V1) and somatosensory (S1) cortices in intact and enucleated C57Bl/6 mice, and to determine quantitative differences in the proportion and laminar distribution of neurons and terminals in these projections. CTB labeled neurons were used to estimate the relative importance of projections between V1 and S1, and their laminar distribution used to classify them as feedback, feedforward or lateral projections. The size of axonal swellings was measured and frequency distribution determined for each cortical layer. Axon diameters were also sampled in these connections. Injections in V1 resulted in a reduced proportion of labeled cells in S1 of enucleated mice. There was a relative decrease of the projection from the somatosensory cortex to the visual cortex in enucleated mice due to a greater relative reduction of supragranular layers neurons. Enucleation otherwise had no effect on the connectivity of the somatosensory cortex with other motor and somatosensory cortices. In the projection from the visual to the somatosensory cortex in enucleated mice, the size of axonal swellings was reduced in all layers. Conversely, in the projection from the somatosensory cortex to the visual cortex, some larger swellings appeared in the supragranular layers. This shows pathway and laminar specific changes in cortical circuitry following loss of sensory afferent activity that results in changes in the relationship between visual and somatosensory cortices in the enucleated mice.

neuroscience↗